What size in line fuse needed

i am installing another, bigger amp to hook up more subs and was told you need a main in line fuse right next to the battery and then two more after the ditrabution block and was wondering if that is correct and what size in line fuse should i use for a 1800 watt amp?

For a safe installation, you need a fuse as close to the battery as possible.
You should also fuse any lines where there is a change in wire size, which usually means after a distro block.

As far as the fuse sizes you need, we can't tell you without knowing the total amperage the equipment will draw.
There are tons of guides to choosing proper fuse ratings on the web. I'm pretty certain there's one on these forums, as well.